1. Field of the Invention
This invention relates generally to a corona ignition system and method for igniting a combustion mixture of fuel and air in a combustion chamber.
2. Related Art
Corona discharge ignition systems provide an alternating voltage and current, reversing high and low potential electrodes in rapid succession which makes arc discharge formation difficult and enhances the formation of corona discharge. The system includes a corona igniter with an electrode charged to a high radio frequency voltage potential and creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. During typical operation of the corona discharge ignition system, the electric field is controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. The corona discharge has a low current and can provide a robust ignition without requiring a high amount of energy and without causing significant wear to physical components of the ignition system. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
It is typically desirable to control the electric field so that the fuel-air mixture does not lose all dielectric properties, which would create a thermal plasma and an electric arc discharge between the electrode and grounded cylinder walls, piston, or other portion of the corona igniter. However, arc discharge oftentimes occurs, either intentionally or unintentionally, due to the high voltage required to produce the corona discharge and also due to changing engine operating conditions. The duration and intensity of the arc discharge in the corona ignition system is typically not great enough to provide reliable ignition of the combustible mixture.